The capabilities of the simple Hi-Fi stereo system have been extended recently to incorporate the surround sound effects required by home theater systems. Such systems include a large-screen television receiver or video cassette player, four additional speakers, and a surround amplifier. The new system dramatically improves the immersion of the viewer in the sound effects of the movie.
A typical home theater system combines video capabilities with advanced audio systems, and it is based on the following major components:    1. A large screen TV receiver or video projector.    2. A laser disk player or a Hi-Fi video cassette player, which is the source of the audio and video signals. The audio track recorded on the film is not an ordinary stereo track. It encrypts additional information about the sound channels. The encryption protocols have evolved over the years. There are three major standards currently in use:            a. Dolby ProLogic Surround in which in addition to the standard left and right channels, a center channel and a rear channel are recorded on the sound track. All channels are analog.        b. THX, manufactured by the Lucas film company, in which two separate rear channels are used instead of one. All channels are analog.        c. AC-3, the latest development by Dolby lab, in which six channels of music are digitally recorded on the sound track—front right, front left, center, rear right, rear left and subwoofer. The latter is not a full spectrum channel, as only one octave is necessary.            3. A surround amplifier, for extracting the surround channels from the incoming signal. Surround amplifiers are typically based on the Dolby chip. Most amplifiers have DSP (Digital Signal Processor) capabilities, which can modify the sound of a non-surround music source to sound as if it originates from different artificial acoustic environments, such as a concert hall, a theater, a jazz club, etc.    4. Speakers. A full surround system requires six different speakers, which must be of high quality to ensure realistic reproduction. Their function is as follows:            a. Two main speakers, which reproduce most of the sound and music effect.        b. One center speaker, located above or below the screen. This speaker is dedicated to the actors' voices.        c. Two rear speakers, responsible for the special effects generated by the surround sound system, and for the artificial echo effects generated in the different DSP modes of the surround amplifier.        d. A subwoofer, for reproducing all low frequency sounds, such as explosions. Location of the subwoofer is not critical, as this channel contains little directional information. Furthermore, such low frequency sound waves are felt by many parts of the body, and not specifically by the ears. The subwoofer is usually placed in the front field.        
The room itself has to be modified to fit the home theater requirements:    a. Since there are six different sound sources in the room, any unwanted echo destroys the sound quality and directionality. The room must therefore be covered with acoustically absorbing materials, such as carpets and drapes.    b. Acoustical isolating materials must be used to avoid disturbing neighbors.    c. Wiring to the various speakers must be installed in the room, preferably without being a visual eyesore.
Each of the system elements affects the overall sound quality. The most important factor is the room acoustics. If the room is big and the walls bare, the echo severely affects the sound. The quality of the speakers is also a major element of the system. High performance speakers are large and expensive, but essential for good sound. Finally, the high power, low distortion amplifiers required for realistic surround sound are expensive.
These requirements make high quality surround sound systems very expensive both to purchase and to install in the home.
In order to provide high quality audio reproduction at low cost and at a personal level of listening, conventional Hi Fi audio systems have for a long time made use of stereo headphones. Attempts to utilize headphones to provide surround sound have been made by a number of manufacturers with limited success. In order to appreciate the problems involved in achieving an effective implementation of surround sound headphone technology, it is necessary to understand the physiological effects used by humans in experiencing three dimensional hearing.
In order to recognize the direction of a sound, the brain combines information received by the two ears and uses several psycho-acoustic effects to achieve a 3-D sensation of the surrounding world, as follows:    1. Phase difference: The sound does not reach both ears in the same phase—the ear closer to the sound source hears the sound first. By calculating the minute differences in time of arrival of the sound at the two ears (<1 msec.), the brain can detect the origin of the sound.    2 Level difference: The ear closer to the sound source hears a louder sound. This information is converted by the brain into directional and range information.    3. Head rotation: If, for example, the sound source is directly in front of or directly behind the listener, the phase and level difference between the two ears is zero. The body executes small, almost unnoticeable head movements in order to identify the origin of a sound. Even the smallest movement creates phase differences significant enough for the brain to discern the orientation of the source.    4. Doppler pitch difference: During head rotation, the sound pitch changes due to the Doppler effect. The ear which rotates towards the source hears a slightly higher pitch than the other one. The brain is capable of detecting this slight change in pitch, and decoding the source direction from this information.    5. Face blockage: While rotating the head away from the sound source, at a certain angle, the listener's head causes one ear to move into the “acoustical shade area” from the sound source, and the sound level in this ear becomes lower than in the other one. The brain uses this effect to locate the sound origin point.
The first three effects are the most important, but in order to get a perfect illusion, all five have to be reproduced correctly. When surround sound is produced by an array of speakers, the sound field produced is very similar to that present in real life, and the human brain is able to make use of all five of the above effects to appreciate the sound.
The use of headphones, however, effectively eliminates all five of the above effects present in free space propagation, since the sound originates from highly localized transducers close to the listener's ears. As the listener moves or turns his head the headphones move together with the listener's head. The use of simple binaural audio signals do not therefore give a perception of realism, since the sound field moves with the listener's head. In order to create a true surround sound effect, the audio signal supplied to the headphones must be coded in a sophisticated manner in order to simulate all five of the above psycho-acoustic effects as the listener moves while listening to the performance or the film.
Japanese Unexamined Patent Publication No. Sho 42-227 and Japanese Examined Patent Publication No. 54-19242 describe a surround sound headphone system including a gyro compass or a magnetic needle compass installed on the headphones to measure head movement and to transmit information about head position to a microprocessor. This microprocessor modifies the sound track signal according to the head angle, and transmits the modified signal back to the headphones, so that the listener experiences a surround sound effect. Such a system, using a gyroscope mounted in the headphones, has been marketed by the Sony Corporation. In U.S. Pat. No. 5,181,248 (corresponding to EP 438281), U.S. Pat. Nos. 5,452,359 and 5,495,534, a further development of this system is described in which the gyroscope is replaced by an ultrasonic ranging system. The angular location of the head is obtained from relative time-of-arrival measurements of an ultrasonic reference signal emitted by a transmitter located in front of the listener, by means of ultrasonic detectors located in the left and right arms of the headphone set. As previously, a microprocessor modifies the sound track signal according to the measured head angle and transmits the modified signal back to the headphones, so that the listener experiences a surround sound effect.
In a further system, developed by Virtual Listening Systems Inc. and described in Stereo Review, p. 38 (April 1997), head movements are ignored completely. The surround sound effects from typical audio situations are pre-programmed by algorithms which provide the phase shifts and volume changes corresponding to various situations. This system therefore simulates the surround sound effect by digital processing means.
German Patent 26 52 101 discloses a device with wireless transmission of a sound signal to a headphone by means of a transmitted carrier. Two reception elements are attached to the pair of headphones so as to afford directing characteristics close to that of human hearing.
European Patent Application No. EP 705053 describes a headphone for surround sound effect having two earphone members, each being provided with at least two loudspeakers arranged facing and forward of, or adjacent to, the pinna of the listener's ear.
All of the above-mentioned prior art systems use advanced real-time signal processing to modify the audio signal information. But the speed of available processors is such that they are unable to process the signals effectively, and the subjective results are unsatisfactory for a number of reasons:    a. The systems deal, only with the main psycho-acoustic parameters affecting 3-D recognition, namely, the first two, or at best three, in the list above. They all ignore the other, usually neglected, yet important, effects of Doppler pitch change effect and face blockage.    b. The relatively slow signal sampling rate results in an unnatural “metallic sound”.    c. The currently available real time computing used is not fast enough. If the listener turns his head too fast, the computing delay is clearly discerned and disturbing.    d. In both the above mentioned commercially marketed systems, RF is used for communication between the headphones and the processor. RF is prone to interference from external sources such as cellular phones, radio transmitters or even a second headphone system nearby. Conversely, RF can interfere with other such systems.    e. The processor can only deal with one set of headphones. In order for a second listener to enjoy the movie, a complete second system needs to be purchased.    f. Because of the complexity of the systems, they are expensive.
Therefore, it would be desirable to provide a headphone surround sound system which overcomes the disadvantages of the prior art technology, in that:    a. It takes into consideration all five physiological aspects of 3-D sound appreciation, to provide perfect surround illusion;    b. It provides excellent sound quality, without any hesitation or metallic-sounding effects;    c. It is useable by several listeners, each listener requiring only a separate pair of headphones, all being controlled by one processing unit;    d. It is reasonably priced, and    e. It does not use interference-prone RF communication channels.